The effects of growth hormone on avian skeletal muscle satellite cell proliferation and differentiation.

Growth hormone receptor (GH-R) mRNA was expressed in avian skeletal muscle tissue and satellite cells in culture, and was capable of binding chicken growth hormone (cGH). In the satellite cells, GH-R gene expression was regulated by cGH in a biphasic manner which correlated with the GH effect on cell proliferation: 2-10 ng/ml of the hormone increased GH-R mRNA and DNA synthesis, whereas higher concentrations attenuated these effects. GH induced insulin-like growth factor I (IGF-I) mRNA, a potential factor for satellite cell proliferation and differentiation. However, GH inhibited the gene expression of myogenin and the expression of muscle-specific proteins in a dose-dependent manner. These results suggest a role of GH for inhibiting satellite cell differentiation in an IGF-I-independent manner. During satellite cell differentiation, both GH-R mRNA expression and cGH binding peaked when cells were still proliferating and beginning to fuse, and then declined as cells fully differentiated. GH-R mRNA expression in muscle tissue and the satellite cell fraction was evaluated during chicken growth. In both fractions, GH-R mRNA peaked at 4 days of age and then declined in correlation with the reduction of muscle regulatory gene expression. Our results are in contrast with previous studies on rat muscle satellite cells, suggesting a difference between mammalian and avian species in the mode of action of GH in these cells. Our notion is that GH, via its own receptor, promotes more satellite cells to proliferate by inhibiting their differentiation, leading to the addition of more nuclei to the growing muscle.